An adhesive agent containing a thermoplastic block copolymer as a main component has been used in disposable products typified by a paper diaper and a napkin and, particularly, a hot-melt adhesive agent based on a styrene class block copolymer has widely been used. For example, a paper diaper is produced by bonding a polyethylene film with other members (for example, a nonwoven fabric, an elastic material such as a natural rubber, a water-absorbing paper, etc.) using a hot-melt adhesive agent. The hot-melt adhesive agent can be applied to various members using various methods and, even when using any method, the hot-melt adhesive agent is melted by heating so as to obtain an appropriate viscosity, and then the molten adhesive agent is applied to various constituent members in a dot, linear, stripe, spiral or sheet form.
It is now required for the paper diaper to improve drapeness thereof, and a study has been made in improving flexibility and drapeness of the paper diaper by more thinning a polyethylene film or the above-mentioned various members such as a nonwoven fabric. Thinning of various members significantly reduces material costs. However, thinning of the polyethylene film may cause a problem that heat resistance deteriorates and application of a high-temperature (not lower than 150° C.) hot-melt adhesive agent leads to melting of the polyethylene film or formation of wrinkles of the polyethylene film. Therefore, adhesive agent manufacturers have made a progress on the development of a low-temperature-applicable hot-melt adhesive agent which is capable of applying at low temperature (not higher than 140° C.).
Taking workability and environmental aspect in the case of application of the hot-melt adhesive agent into account, manufacturers producing a paper diaper and a sanitary good strongly desire lowering of the viscosity of the hot-melt adhesive agent. The hot-melt adhesive agent commonly comprises a base polymer and a plasticizer, and a study has been made in lowering the viscosity of the hot-melt adhesive agent by a method in which the amount of the base polymer is decreased to thereby increase the amount of the plasticizer. However, the production of a paper diaper using a low viscosity hot-melt adhesive agent produced using such method may cause a problem that the balance between an adhesiveness to a polyethylene film which composes members of the paper diaper and a retention force (cohesive force) is deteriorated, and the softening point is excessively lowered.
JP 2004-137297 A discloses a hot-melt adhesive agent including a linear type styrene block copolymer, a tackifier resin and a plasticizer (claim 1). The hot-melt adhesive agent of the literature has a low viscosity, and suitable for applying at low temperature, however, is not sufficient in adhesiveness at low temperature in winter or at high temperature in summer.
JP H5 (1993)-311138 A mentions a hot-melt adhesive agent including a radial type styrene block copolymer (claim 1). However, the hot-melt adhesive agent of the JP H5 (1993)-311138 A has high melt viscosity, and are not suitable for conducting application at low temperature. The hot-melt adhesive agent is insufficient in peel strength at 40° C.
In JP 2009-511713 A, structure of the block copolymer or melting point of the tackifier resin is specified, and combination ratio of the both is further adjusted to obtain a hot-melt adhesive agent suitable for coating at low temperature (claim 1). The hot-melt adhesive agent of JP 2009-511713 A is, however, insufficient in adhesiveness at low temperature, low in tackiness, and insufficient in balance between retention force and tackiness. The hot-melt adhesive agent of JP 2009-511713 A, therefore, does not completely satisfy the high level requirements of users for use in disposable produces.